A cholesterol oxidase is an oxidoreductase which acts on cholesterol and oxygen molecule as a substrate. Measurement of cholesterol using a cholesterol oxidase is often carried out in clinical tests. A method for measuring cholesterol in a specimen by a colorimetric method or the like is used in clinical tests. In such a colorimetric method, the cholesterol oxidase acts on cholesterol in a specimen to form hydrogen peroxide, the hydrogen peroxide formed is reacted with an oxidative-coloring chromogen in the presence of a peroxidase to convert the chromogen to a dye, and the absorbance of the dye formed is measured.
A cholesterol oxidase used for measuring cholesterol is unstable. The problem of the cholesterol oxidases is that the cholesterol oxidase is deactivated during preservation of a reagent for measuring which contains the cholesterol oxidase to give a reduced performance of the reagent for measuring.
A method for stabilizing a cholesterol oxidase, wherein alkali metal chloride and/or alkaline earth metal chloride are added to a solution containing the cholesterol oxidase (see patent document 1), a method for stabilizing a cholesterol oxidase in solution, wherein the cholesterol oxidase is chemically bound to a water-soluble carrier selected from bovine serum albumin, dextran, and polyethylene glycol (see patent document 2), a method for stabilizing a cholesterol oxidase by adding of bovine serum albumin and lysine to a solution containing the cholesterol oxidase (see patent document 3), a method for stabilizing a cholesterol oxidase in a dried state by allowing the cholesterol oxidase to coexist with protein decomposition products (see patent document 4), and the like are known for this problem.